The present disclosure relates to turbochargers having a sliding piston in the turbine nozzle for regulating exhaust gas flow into the turbine.
An exhaust gas-driven turbocharger is a device used in conjunction with an internal combustion engine for increasing the power output of the engine by compressing the air that is delivered to the engine's air intake to be mixed with fuel and burned in the engine. A turbocharger comprises a compressor wheel mounted on one end of a shaft in a compressor housing and a turbine wheel mounted on the other end of the shaft in a turbine housing. Typically the turbine housing is formed separately from the compressor housing, and there is a center housing connected between the turbine and compressor housings for containing bearings for the shaft. The turbine housing defines a generally annular chamber that surrounds the turbine wheel and that receives exhaust gas from the engine. The turbine assembly includes a nozzle that leads from the chamber into the turbine wheel. The exhaust gas flows from the chamber through the nozzle to the turbine wheel and the turbine wheel is driven by the exhaust gas. The turbine thus extracts power from the exhaust gas and drives the compressor. The compressor receives ambient air through an inlet of the compressor housing and the air is compressed by the compressor wheel and is then discharged from the housing to the engine air intake.
One of the challenges in boosting engine performance with a turbocharger is achieving a desired amount of engine power output throughout the entire operating range of the engine. It has been found that this objective is often not readily attainable with a fixed-geometry turbocharger, and hence variable-geometry turbochargers have been developed with the objective of providing a greater degree of control over the amount of boost provided by the turbocharger. One type of variable-geometry turbocharger employs a sliding piston in the turbine nozzle. The piston is slidably mounted in the turbine housing and is connected to a mechanism that translates the piston axially back and forth. Changing the position of the piston has the effect of changing the effective flow area through the turbine nozzle, and thus the flow of exhaust gas to the turbine wheel can be regulated by controlling the piston position. In this manner, the power output of the turbine can be regulated, which allows engine power output to be controlled to a greater extent than is generally possible with a fixed-geometry turbocharger.
Typically the sliding piston mechanism also includes vanes that are either attached to an end of the piston or to a fixed wall of the turbine nozzle. When the piston is fully closed, there is still an opening between the end of the piston and the fixed wall of the nozzle, and the vanes typically extend fully across this opening. However, when the piston begins to open, in some such piston mechanisms a vane-free gap begins to develop either between the end of the piston and the ends of the vanes (when the vanes are mounted on the fixed nozzle wall) or between the ends of the vanes and the nozzle wall (when the vanes are mounted on the piston). This is undesirable because at the moment the gap begins to develop, the flow of exhaust gas around the vane ends and through the vane-free gap has poor aerodynamics, which adversely impacts turbine efficiency. The flow rate into the turbine also tends to change quite abruptly with small changes in piston position during this initial opening movement of the piston, which makes it difficult to control the turbine with accuracy during this transition.
In order to try to overcome such disadvantages, it has been proposed to include slots either in the piston end or in the nozzle wall for the vanes to extend into. In this manner, the vanes can be made long enough so that even when the piston is fully open, the vanes extend fully across the nozzle opening. However, this has its own drawbacks. Because the exhaust gas flowing through the nozzle is very hot, the piston, vanes, and nozzle wall are all subject to dimensional changes caused by thermal growth and contraction as the gas temperature changes. Accordingly, in order to prevent the vanes from binding in the slots at all operating conditions, it is necessary to provide large tolerances. Therefore, there are substantial gaps between the vanes and the edges of the slots that receive the vanes, and the exhaust gas can leak through these gaps. This not only partially defeats the purpose of the vanes, but when the slots are in the fixed nozzle wall they can allow hot exhaust gas to migrate into the center housing where the gas can heat up the bearings, which is highly undesirable.